In typical railroad control systems, a length of many miles of track may be divided into a plurality of successive adjacent blocks. When a block is too long to allow satisfactory operation with a single track circuit, the block is subdivided into a plurality of cut sections with a track circuit for each cut section. For the purposes of this description, it will be assumed that it is not necessary to subdivide the blocks into cut sections. However, it should be understood that the track circuit described herein will function with cut sections just as it does with blocks.
The track circuit provides means for detecting the presence or absence of a railroad vehicle in a given block. The information thus obtained is used for traffic control in allowing trains to operate at safe speeds and to identify their locations as they pass from one block to another. One method of distinguishing between the plurality of blocks of the system is to provide a means for electrically insulating the tracks of one block from the tracks of an adjacent block. That is, during the construction of the rail system, each rail of the double rail track is provided with an electrical insulator at suitable intervals. Accordingly, an electrical signal applied to one rail will be confined to one block because of the electrical insulation which isolates that rail from the adjacent blocks. It should be understood that when cut sections are used, there is insulation between rails of adjacent cut sections.
A wide variety of non-standard conditions and/or faults may result in a broken down insulation such that a signal applied to the rail of one block may be conducted to the rail of an adjacent block or blocks. Obviously, such failures may result in the loss of supervision over the railroad system and inaccurate identification of railroad vehicle location within the block system.
Both a.c. and d.c. track circuits have been used in the past and both have advantages and disadvantages. The track circuit disclosed herein is an a.c. track circuit. It is known that high frequency and low frequency track circuits each have advantages and disadvantages. More specifically, a low frequency track circuit works over substantial lengths, but is subject to interference from parallel low frequency circuits such as power lines and/or rail propulsion currents. High frequency track circuits are much less sensitive to induced interference but function over a more limited length and thus require an increased number of track circuits for a given total length of track.